Two-step extraction for the evaluation of metal-organic framework impregnated materials

Metal-organic frameworks (MOFs) are widely used for gas adsorption, separation, and sensing materials. In most cases, MOFs are not used in their crystal form but as impregnated materials because the fine crystals result in high-pressure drops. One key characteristic of MOF-impregnated materials is the amount of MOF in the material. This is evaluated using the wet digestion method; however, it is limited to determining only the metal content. Moreover, some metal, denoted as free metal, will not react with ligands to form MOFs. Additionally, it is crucial to determine the ligand amount, which cannot be determined using wet digestion. In the present study, a two-step extraction method for copper (II) benzene-1,3,5-tricarboxylate (Cu-BTC MOF) impregnated materials was developed to determine the MOF formed and free metals and ligands. Various solvents were applied to evaluate the extraction efficiencies. The results led to the selection of ethanol (EtOH) for extracting free Cu²⁺ and BTC, while 0.3 M HNO₃ was chosen to extract MOF-formed Cu²⁺ and BTC. The MOF-impregnated sample material was first extracted using EtOH and then 0.3 M HNO₃. The Cu²⁺ and BTC in the obtained extract solutions, as well as EtOH and HNO₃, were analyzed using flame atomic absorption spectroscopy and high-performance liquid chromatography, respectively. In standard addition tests, free and MOF-formed Cu²⁺ and BTC were quantitatively extracted from MOF-impregnated materials. The developed two-step analysis method was successfully applied to Cu-BTC-impregnated materials used in gas sensing.